Say, I'm not the only who's really annoyed by Sugiyama as a character, am I? The lack of character development, her unwavering obsession with ruining a perfectly stable and happy marriage just to validate her horribly cynical ideology regarding the institution of marriage, and worst of all never suffering any real repercussions for her immoral attempts at marriage-sabotage... really, she's the closest to being an actual villain that this story has.
At least Makoto's two make coworkers, despite being male-chauvinistic jerks and perverts who ogle Makoto's wife on any chance they get, don't actively try to ruin the couple's marriage, and do get punished with proportional humiliation to what line-crossing jerkass acts they do occasionally commit.